An acoustic transducer is an electronic device used to emit and receive sound waves. Ultrasonic transducers are acoustic transducers that operate at frequencies above 20 KHz, and more typically, in the 1-20 MHz range. Ultrasonic transducers are used in medical imaging, non-destructive evaluation, and other applications. The most common forms of ultrasonic transducers are piezoelectric transducers. In U.S. Pat. No. 6,271,620 entitled, “Acoustic Transducer and Method of Making the Same,” issued Aug. 7, 2001, Ladabaum describes capacitive microfabricated transducers capable of competitive acoustic performance with piezoelectric transducers. Such transducers have advantages over piezoelectric transducers in the way that they are made and in the ways that they can be combined with controlling circuitry, as described in, for example, U.S. Pat. No. 6,246,158, issued Jun. 12, 2001 to Ladabaum.
The basic transduction element of the conventional microfabricated ultrasonic transducer is a vibrating capacitor. A substrate contains a lower electrode, a thin diaphragm is suspended over the substrate, and a metallization layer serves as an upper electrode. If a DC bias is applied across the lower and upper electrodes, an acoustic wave impinging on the diaphragm will set it in motion, and the variation of electrode separation caused by such motion results in an electrical signal. Conversely, if an AC signal is applied across the biased electrodes, the AC forcing function will set the diaphragm in motion, and this motion emits an acoustic wave in the medium of interest.
Microfabricated transducers are typically made on flat, rigid substrates, as required by microfabrication equipment. However, transducers with curvature are desirable in many applications. In fact, at least half of all practical medical ultrasound probes use curved transducer arrays. A typical range for the radius of curvature of an abdominal array is 4 to 6 cm, though trans-vaginal and other probes can have even smaller radii of curvature.
FIG. 1 illustrates the naming conventions of orientation and direction used in ultrasound engineering. As shown in FIG. 1, the transducer 100 is typically made up of multiple transducer elements 110. Each of the transducer elements 110 includes a plurality of individual transducer cells. The transducer elements 110 are oriented such that their lengths are along the elevation axis, and their widths are along the azimuth axis. The transducer elements 110 are adjacent to one another along the azimuth axis.
Physical curvature is desirable in diagnostic medical ultrasound to improve image field of view in the imaging plane as well as to provide for out-of-plane focusing (i.e., elevation beam-width control). It should be noted that the literature often refers to electronic delays as creating transducer curvature. But the real, physical curvature addressed herein is different than, and should not be confused with, this virtual, electronic curvature.
Currently, the most common forms of ultrasound imaging systems generate images by electronic scanning in either linear format or sector format. FIG. 2 illustrates the linear 210 and sector 220 image formats generated by a typical ultrasound system. As shown in FIG. 2, in linear format 210 scanning, time delays between transducer elements are used to focus the ultrasound beam in the image plane. Also shown in FIG. 2, in sector format 220 scanning, time delays between transducer elements are used both to focus the ultrasound beam and to steer it. Typically, the sector scan format 220 is used to image a relatively large, deep portion of the anatomy from a small acoustic window (e.g., imaging the heart); whereas the linear scan format 210 is used for optimum image quality near the face of the transducer (e.g., imaging the carotid). For a similar frequency range, the system and transducer requirements for sector scanning are more challenging than those required for linear scanning. In order to beam-steer, as is required in the sector format 220, the transducer elements of an imaging array need to be small enough to provide for an adequate acceptance angle, and cross-talk between channels needs to be kept to a minimum. In linear format 210, the restrictions on transducer dimensions, system and transducer cross-talk, and the dynamic range of the beam-former's timing are more relaxed.
Historically, the differences in these technological challenges led to the linear format 210 being preferred, if possible, over the sector format 220 by ultrasound system manufacturers. To provide the advantages of the sector format 220 field of view, but still maintain the system simplicity of the linear scan format 210, curvilinear transducers (i.e., linear transducers with convex curvature in the azimuth direction) were introduced in the art. The curvilinear transducers can be used in the linear scan format 210 for deep and wide scanning when a relatively large acoustic window is available (i.e., abdominal as opposed to cardiology imaging).
With the advent of digital beam-forming, system complexity is no longer the primary motivator for curved arrays; but physical curvature is nevertheless still desirable because it leads to superior image quality in a variety of applications. Note in FIG. 2 that there are significant regions 230, near the face of the transducer, where the sector format 220 does not interrogate. A curvilinear transducer would image this region. Thus, a curvilinear transducer employing the linear scan format is better suited for situations where both near field and wide angle fields of view are desirable. Furthermore, one would prefer to use the largest anatomically feasible aperture to form an image, while at the same time keeping system channel count to a reasonable number. Curvilinear transducers, because they do not need to beam-steer, are larger for a given channel count and field of view than sector transducers, and are thus able to produce higher quality images.
Curvilinear piezoelectric arrays are more difficult to assemble than conventional non-curved arrays because the piezoelectric ceramics are not flexible. Convex, piezoelectric, curvilinear arrays are disclosed in U.S. Pat. No. 4,344,327, issued Aug. 17, 1982 to Yoshikawa et al., and concave curvilinear arrays are disclosed in U.S. Pat. No. 4,281,550, issued Aug. 4, 1981 to Erikson. These patents teach methods of dicing and re-assembling piezoelectric arrays so that the advantage of performing sector fields of view is made possible without the need for electronic sector scanning techniques to steer the ultrasonic beams over large angles. Common to all of the teachings is a combination of dicing through the rigid transduction material and re-assembly methods such that the re-assembly of the diced elements into a curved structure is practical.
Furthermore, azimuth curvature is not the only desirable curvature of medical ultrasound probes. Elevation curvature is desirable to achieve elevation beam focus without the need of lossy lensing material. FIG. 3 illustrates elevation focusing as provided by a lens on a typical ultrasound probe. As shown in FIG. 3, typically, lensing material 120 is used to achieve the focus 130 of element 110A of transducer 100. U.S. Pat. No. 5,423,220, issued Jun. 13, 1995 to Finsterwald et al., teaches piezoelectric transducers with concave elevation curvature for focus and convex azimuth curvature. U.S. Pat. No. 5,415,175, issued May 16, 1995 to Hanafy et al., teaches, among other things, that piezoelectric transducer curvature in elevation is desirable to eliminate the generation of reflections from the face of the transducer that can lead to reverberation artifacts.
Physical curvature is also desirable in therapeutic ultrasound probes. Physical curvature focusing of the transducer could eliminate the necessity of electronic focus, which is challenging at the high power levels of therapeutic probes. Also, physical curvature focusing could eliminate the uses of focusing lenses, which are lossy and can generate excessive heating of the therapeutic probes.
Thus, it is desirable to provide for capacitive microfabricated ultrasonic transducers with curvature, such that the benefits and advantages of curvature, many already known and taught in the prior art for piezoelectric transducers, can be imparted to microfabricated transducers.
In co-pending U.S. patent application Ser. No. 09/435,324 filed Nov. 5, 1999, Ladabaum describes microfabricated transducers with polyimide structures on the front of the transducer and notches through the substrate to such walls in order to, among other things, make the transducer flexible. The teaching and structure in the '324 application describe a transducer that could have curvature in azimuth plane, though such a curved transducer is not specifically taught or claimed. Furthermore, it is not clear how such a method could provide for transducers with both elevation and azimuth curvature. Common to this and the cited piezoelectric prior art is that dicing is necessary for azimuth curvature. In the piezoelectric case, elevation curvature can be achieved either by dicing (i.e., Finsterwald) or by starting the transducer fabrication by providing for plano-concave (i.e., Hanafy) or otherwise rigidly formed and curved piezoelectric substrate. It is therefore desirable to have microfabricated transducer structures with concave or convex curvatures in azimuth, in elevation, or in both azimuth and elevation planes which can be easily formed.
It has been realized by the present inventors that a silicon substrate with microfabricated transducers on its surface, lapped or otherwise thinned to suitable dimensions can result in microfabricated ultrasonic transducer elements and arrays that are sufficiently flexible to create curved ultrasound probes.
In co-pending U.S. patent application Ser. No. 09/971,095 filed Oct. 19, 2000, Ladabaum et al. teach that substrate modes in the silicon substrate of microfabricated ultrasonic transducers exist, and that effective ways of damping such substrate modes include backing the transducer, thinning the transducer, and a combination of backing and thinning the transducer. U.S. Pat. No. 6,262,946, issued Jul. 17, 2001 to Khuri-Yakub et al., describes microfabricated ultrasonic transducers with substrate thinned such that the critical angle of a lamb wave mode is outside of the acceptance angle of interest. Neither Ladabaum nor Degertekin teach the flexible properties of a thin substrate or any application of thinning beyond that of substrate mode control and damping.
Flexible acoustic transducers are known in the art that are able to take curved shapes. Piezoelectric polymers, such as polyvinyl difluoride (PVDF) have been used for decades. The piezoelectric properties of such polymers, however, are not advantageous for conventional medical imaging, and thus have not been successfully applied to medical imaging. Canadian Patent No. 1,277,415, issued Dec. 4, 1990 to Clark et al., discloses an elastomeric electrostatic transducer that is flexible. However, this transducer is effective in the audible range, not at the ultrasonic frequency range of interest in medical ultrasound applications, and the techniques used in its fabrication cannot yield efficient transducers in the MHz range. For example, for useful ultrasonic transducers, vacuum gaps, not elastomeric structures with gas bubbles, are needed between the electrodes, and the gap dimensions needed for ultrasonic transducers are on the order of 0.1 um, far smaller than those taught in the Clark patent.
Thus, what is needed is a microfabricated ultrasonic transducer with acoustic performance in the MHz range, with physical curvature, and a simple and practical method of achieving such curvature. The present invention provides such a transducer.